White-light coronal dark threads and density fine structure

High spatial resolution white-light coronal observations from the eclipse of 1991 July 11 taken with the 3.6 m Canada-France-Hawaii Telescope on Mauna Kea are the best ever achieved because of the unique opportunity of observing the totally eclipsed Sun with a large aperture telescope. Photometric calibration of the photographic data is verified by the average radial intensity variation, which agrees with the classical function. Dividing out the average radial variation, we identify relatively dark and bright fine ''threads'' with enhanced spatial power in the range of 1-5 Mm. Spatial filtering using unsharp masking or ''mad-max'' algorithms clearly shows relatively dark and bright radial threads in loops as fine as the resolution limit of about 1 Mm. The main feature is an arcade of concentric dark and bright threads that extend above a small prominence just above the limb. A small coronal cavity near the prominence contains numerous fine dark threads. The relative electron-density depletion and enhancement required to explain the observed thread contrast, assuming that they are isolated cylindrically symmetric structures, is found to be Delta N-e/N-e similar to +/- 100%, indicating that the dark threads are fully evacuated. An excess of approximately 1 G field strength in stationary non-force-free magnetic fields can produce the observed thread contrast. Evacuated threads representing non-force-free magnetic fields introduce special constraints on the coronal thermodynamics.